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ABSTRACT

Failure-prone Cenozoic volcanic rocks distributed across central Idaho, USA, promote large landslides, consequent drainage impoundment, and the formation of regionally asynchronous landslide-dammed lakes. Examination of sedimentary records shows that extant lakes formed in this way exhibit high primary productivity relative to other lakes in the region, apparently sustained through relatively elevated watershed phosphorous loading and the contribution of nitrogen-fixing cyanobacteria. The resulting high rates of sediment and carbon accumulation exceed those found in regional lakes formed by other processes and underlain by other bedrock lithologies lower in phosphorous. These unusually high biogenic sediment accumulation rates produce highly resolved, often annually laminated sedimentary sequences. The result is a high-resolution temporal matrix for the runoff-intensity signal of episodically delivered, watershed-derived clastic sediment. Elemental analysis by core-scanning X-ray fluorescence (XRF) effectively highlights these clastic pulses, and spectral analysis of lithogenic elemental intensities indicates they carry spectral power (including significant harmonic signals) concentrated in the 3–5 yr period. Patterns shown by episodic sediment delivery events support winter snowpack as a modulator of late Holocene sediment export from these watersheds.

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